A multi-analytical characterization of fourteenth to eighteenth century pottery from the Kongo kingdom, Central Africa

Pottery traditions reflect the socioeconomic framework of past cultures, while the spatial distribution of pottery indicates exchange patterns and interaction processes. Material and earth sciences are employed here to determine raw material sourcing, selection and processing. The Kongo kingdom, internationally renowned since the late fifteenth century, is one of the most famous precolonial states in Central Africa. Despite the large number of historical studies relying on African and European oral and written chronicles, there are still considerable gaps in our current understanding of this political unit. Here, we provide new insights into pottery production and circulation within the Kongo kingdom. Implementing a multi-analytical approach, namely XRD, TGA, petrographic analysis, XRF, VP-SEM-EDS and ICP-MS, on selected samples, we determined their petrographic, mineralogical and geochemical signatures. Our results allowed us to correlate the archaeological objects to naturally occurring materials and to establish ceramic traditions. We identified production templates, exchange patterns, distribution of high-quality goods and interaction processes through technological knowledge transmission. Our results demonstrate that political centralisation in the Lower Congo region of Central Africa had a direct impact on pottery production and circulation. We expect our study to provide a sound basis for further comparative research to contextualise the region.


Supplement 1. The Kongo kingdom, Historical information
Based on the reconciliation of oral traditions, Thornton [17,18] proposes the starting point of the Kongo kingdom to be 1350-1375 AD, and suggests that Mbanza Kongo as was conquested by the kingdom's founders. Since its foundation until the late 17 th century, the Kongo kingdom expanded significantly by the integration of new provinces, which were ruled by royal members under the direct administration of the king, except for the Mbata province and in later times the Soyo province [15,17,18,31]. The Kongo kingdom's social structure is characterised by a distinction between towns (mbanza) and villages (mavata) [15,[17][18][19][20]30,31]. Towns were populated by the rulers, the nobles, and the slaves; the slaves contributed to the increase of population in the towns, a crucial feature of the kingdom's economy [19,27,30]. The villages, most abundant in the kingdom, were populated by commoners, most often of local descent [19]. This social division was further extended to the economic processes within the kingdom which had a centralised agricultural system. The management of production (land use, labor, and distribution) and taxation (either in kind, production surplus, specialised products of the rural regions or in money, such as nzimbu shells, palm cloth) also point towards a high degree centralisation within the kingdom [19,20,30].
Exchange of indigenous products stimulated the exploitation, production and manufacture of resources. Thus, the trade network was established on the basis of the local availability and Mbanza Kongo was the hub of this royal monopoly [20]. The expansion of the trade was an outcome of the contact with Portuguese in the late 15 th century and was furthered by the existence of a common currency. Based upon the precontact networks, a long-distance trade was established, and transatlantic contacts were developed [19][20][21][22]25]. Subsequently, European material culture entered into the exchange system [19,20,24]. Another outcome of the Portuguese contact was the introduction and spread of Christianity [20,25]. The Christian religion was adapted to the existent religion and was established as a royal cult [20,25]. The conversion of the kingdom's elite to Christianity reinforced its relationships with Portugal [25].
It was only in the first half of the 17 th century that the trade patterns were changed and the use of the trade routes that pass through Mbanza Kongo was reduced due to the outbreak of civil wars in 1665 [19,20]. These changes and their further effects, such as the evolution of Soyo into an independent province in 1636, the development of alliances between clans, and between provinces from inside and outside of the kingdom (Kimbazu and Kinlaza), as well as the increase of their control over regions, had an adverse impact on the political stability of the kingdom. The reduction of the central power resulted in a decrease of the population's mobility from the towns to the rural regions and the end of the central administration [19,20,22,27,30]. The gradual decline of Mbanza Kongo and of the kingdom led to its gradual incorporation into the Portuguese colony of Angola during the 19 th century, marked by the signature of its vassalage treaty to the king of Portugal in 1861 [15,19].

Supplement 2. Archaeological information
Mbanza Kongo [66,67] The archaeological site of Mbanza Kongo (06° 16' 080 S; 14° 14' 590 E) is on a hilltop of 570m height, in the province of Zaire, in northwestern Angola. The site was the capital of the Kongo kingdom. It was excavated in three campaigns in 2011, 2013 and 2014 by a collaboration of Angolan, Portuguese and Cameroonian archaeologists, within the framework of the project of the Angolan Ministry of Culture for the inscription of the site on the World Heritage list. The excavations were carried out in seven different archaeological stations Bairro Madungu, Tadi dia Bukikwa, Mpindi a Ntadi, Lumbu, Bairro Alvaro Buta, Kulumbimbi, and the Catholic Mission. A survey permitted on an eighth station to document a thick wall of 42,40m length, constructed of laterite blocks, which could be interpreted as the protection wall of the royal palace compound or of a church. The human occupation of the area, as evidenced by archaeology, ranges from the end of the 15 th to the 19 th century. Excavations identified part of a settlement dated to the 16 th century (Bairro Madungu), an architectural structure that could be interpreted as the Jesuit convent dated to the first half of the 17 th century (Tadi dia Bukikwa), an area, dated to the 17 th century, interpreted as the fumigation place of the kings (Mpindi a Ntadi), a stone building that according to oral traditions, is associated with the royal gathering place and is dated to the second half of the 17 th century and the 19 th century) (Lumbu), part of a cemetery with several graves studied, probably associated with a church, in use in the 19 th century (Bairro Alvaro Buta), the Mbanza Kongo cathedral and several burials dated to the 19 th century with children graves outside of the church and a tomb of a young woman inside of it (Kulumbimbi) and at last a living area within the Catholic Mission dated to the 17 th -19 th century. Kindoki [30,35,36] Kindoki archaeological site (05° 04' 069 S; 15° 01' 403 E) is located on a hilltop of 580 m height, in the eastern part of the Kongo Central Province of the DRC. The site is associated with Mbanza Nsundi, the capital of Nsundi province. The archaeological excavations, carried out on the Kindoki hill in three campaigns between 2012 and 2015 by the Kongoking project, uncovered an area of 537 m 2 , following mainly a 50-m grid system of 1m 2 squares. The human occupation of the area ranges from the 14 th to the early 20 th century. Excavations in Kindoki revealed three settlements (dated to the 14 th century, 16 th -17 th centuries and late 17 th to the 20 th century) and a cemetery, dated from the late 17 th to the early 19 th century, with eleven graves constructed and oriented the same way, and very closely set up one to the other. Their position together with the funerary goods is interpreted as being burials of the nobles and their associated kinships. Kindoki, as it is confirmed by radiocarbon dating, displays an evidence of continuous occupation from the 14 th to the 20 th century. Ngongo Mbata [31,32] The archaeological site of Ngongo Mbata (05° 47′ 081 S; 15° 07′ 026 E) is located on a hilltop of 823 m height, north of the DRC-Angolan border. The site of Ngongo Mbata is linked historically with the city of Ngongo Mbata or Gongo de Batta, a distinct town of Mbanza Mbata. The first archaeological research on Ngongo Mbata hilltop was carried out between 1938-1942, undertaken by a Belgian archaeologist and priests. Recently, four archaeological campaigns were carried out between 2012 and 2015 by the Kongoking project. During the excavations an area of 847.5 m 2 was uncovered, following mainly a 50-m grid system of 1m 2 squares. The human occupation of the area ranges from the 16 th to the late 18 th century, with a high peak in the 17 th century. Archaeological research in Ngongo Mbata (1938)(1939)(1940)(1941)(1942) revealed a stone building, interpreted as the church of Ngongo Mbata (built between 1633 and 1667, recent radiocarbon dating), and its associated cemetery set up inside. Further excavations (2012)(2013)(2014)(2015) in the area revealed several structures, including the foundations of a small house, possibly attributed to the priest's residence, an iron production area dated to the late 16 th century, a stone smoking pipe workshop area, a stone structure, probably supporting a large wooden cross and the remains of three open-air cemeteries, in use during the 17 th and 18 th centuries. The church cemetery contained high-ranking burials as indicated by the funerary goods. Ngongo Mbata, as it is confirmed by radiocarbon dating, demonstrates that human occupation in the area was dominant from the 16 th to the late 18 th century, with a high peak in the 17 th century.              Sr (ppm)

Rb (ppm)
Type A Type C Type D
The ceramic fragments were initially sub-sampled and subdivided in two parts. The one part was used to obtain a powdered sample to be analyzed by inductively coupled plasma mass spectrometry (ICP-MS) for minor and trace element content identification, by X-Ray diffraction (XRD), either as powder for determination of the bulk mineralogical composition or in the form of oriented aggregate mounts for clay mineral identification, by thermogravimetric analysis (TGA) as powder for identification of specific mineral phases and further prepared to obtain a glass disk, required for X-Ray fluorescence spectroscopy (XRF) for major and minor elements determination. The other part was used to prepare a thin section for the petrographic analysis and variable pressure scanning electron microscopy coupled to energy dispersive X-Ray spectroscopy (VP-SEM-EDS).
ICP-MS, XRD, TGA, XRF: The sample's surfaces were cleaned mechanically, using a multi-tool with a diamond wheel point 4,4mm to remove soil residues and eliminate any contamination. The samples were rinsed using ultrapure water and dried at 40 °C for 24 h. Afterwards, the samples were ground up with an agate mortar and pestle to a fine homogeneous powder approximately 60μm grain size. Ultrapure water and HNO3 (at 6.5%) were used to clean the diamond wheel point and the mortar/pestle between the samples to avoid any contamination by traces.
Prior to ICP-MS analysis, the powdered samples (100mg) were first digested by acid attack on a hotplate. In perfluoroalkoxy (PFA) closed beakers, the selected samples were digested with a mixture of 0,5mL of concentrated HNO3 (65%) and 2 mL of HF (47%), for 48 hours at 140-150°C for silicates digestion. Following cooling, the samples were dried (not completely, to avoid fluorides precipitations and stabilisation) by evaporation on a hotplate. Then, the samples were re-suspended in 2mL of freshly prepared aqua-regia solution [HNO3 and HCl (1:3)] to digest under reflux for 24 hours on a hotplate at 120-140°C. Following cooling, the samples were dried once again on a hotplate. The last digestion step included 2 mL of HNO3 (65%) for 24 hours on a hot plate at 120-140°C with closed beakers. Due to the presence of organic matter, H2O2 (30-32%) was added and the samples were allowed to complete the digestion. Following drying, the samples were finally re-suspended with 1.6 mL of 65% HNO3 and 3 mL of ultrapure water and after transferred to PFA volumetric flasks and fulfilled with ultrapure water up to 50 mL, to reach a solution matrix of 2% HNO3. The samples were diluted 100 fold for minor elements with 2% HNO3 at a final volume of 10 mL. For trace elements determination the samples were analysed with no dilution.
The powdered samples were analyzed by XRD for the determination of the bulk mineralogical composition. The ones displaying peaks in clay-minerals were analyzed as oriented aggregate mounts, after being prepared as follows. The powdered samples were mixed with ultrapure water and after settling for fifteen seconds, the material was extracted by using a pipette and applied on a glass substrate, enabling the clay minerals to lie flat. This procedure was performed in duplicates. Then the oriented aggregate mounts were analyzed dried in room temperature. The samples were subjected into standard treatments, (1) glycolation with ethylene glycol in the oven in 60°C for 12 hours and (2) heating to 400°C and to 550°C, placing the samples in the oven at increasing temperature for fifteen minutes and at 400°C and 550°C, respectively, for thirty minutes. XRD analysis was performed on the non-treated samples and after each treatment.
TGA was performed using a 25-35 mg powder of each of the selected samples.
The powdered samples were further prepared for XRF analysis. A 1,2g of each powdered sample was mixed with 12 g of fusion flux (Li-tetraborate) (Lithium Tetraborate 49.75% (Li2B4O7)/ Lithium Metaborate 49.75% (LiBO2)/ Lithium Iodide 0.5%). The powder mixture was fused in 1065°C and then cast into a glass disk by using a fusion instrument.
Petrographic analysis, VP-SEM-EDS: Thin sections were prepared for petrographic and VP-SEM-EDS analysis. For the preparation, the samples were cleaned with a plastic brush to remove the soil residues and washed using ultrapure water. They were allowed to dry at 40 °C for 24 h. Then, they were embedded in epoxy resin (25:3) (Epofix Fix, Struers A/S; 24 h. hardening time). The cross-sections were then polished using Silicon Carbide papers of different grain sizes (Struers, FEPA P # 320, 500, 800, 1200, 2000 and 4000, where 4000 is 5μm). Afterwards, the cross-sections were glued on a glass with epoxy resin and cut with the saw to a distance of around 1.70cm from the glass. The thin-sections were ground smooth by the saw and then, they were polished using a rotation disk and Silicon Carbide powder (P # 400, 1000), in order the sample to reach the required thickness of around 30μm.